[new][based on reverted project]admin can send join link to new instructors

1. redo the changes to the reverted project
2. now by default sample data will be imported for new instructor
3. modify the joinCourseForInstructor method to handle both instructors added by admin and other instructors
Professor @damithc  this method was checking whether the nominal user has the instructor privileges so I think this should be changed to "instructor".